There is one book in my opinion that stands above the rest. The name of the book is "Story", by Robert McKee. His approach to screenwriting is that it is about form, not formula. He does a fantastic job of helping you understand three act structure.

I've read a lot of books on screenwriting--perhaps about 50--and my latest conclusion is they're all next to useless*. Instead, I'd recommend books about fictional writing in general, or even the creative process in general. A recent winner is Norman Mailer's The Spooky Art: Some Thoughts on Writing. A rambling but at least insightful craft primer is John Gardner's The Art of Fiction.

If you're really interested in the technical minutiae of screenplay form, or how to manifest a particular visual effect into language, then reading books about screenplays is akin to an Olympic athlete training by reading books on pole vaulting. Read screenplays. Lots of them. Drew's Script-o-Rama is one convenient repository.

If you're interested in mythic structure/symbolism, skip Vogler, skip Campbell, and go straight to Diel, who was beloved by even Einstein. Difficult to read, but well worth decoding.

*The old standard Adventures in the Screen Trade by William Goldman achieves exception, but then, it's not really about screenwriting, until it comes to one of the final sections in which Goldman adapts his own short story, which is a really instructive little exercise. And Goldman's thesis, nobody knows anything, applies to our discussion. Nobody knows how to write a good screenplay any more than anybody knows how to write a good novel or compose a good song--author and audience alike only know them when they see them. So then, why should any book be able to tell you how to write a good screenplay any more than how to enjoy a good movie?

Good points by both Paul and Robert. I am of the opinion that you CAN learn from other peoples mistakes as well as successes... so reading screenplays and reading ABOUT screenplays can be beneficial. Why re-invent the process if you can stand on the shoulders of giants?

I've also got a shelf full of books, some excellent, some almost counter-productive. If I had to throw out all of them except two... it would be Volglers and Viki Kings. (Also Goldman's "WHich Lie Did I Tell?" is a roaring good read and a supplement to "Adventures...")

Creativity is such a subjective process, it's hard to recomend a particular path for any one person. That's why it's good to see how MANY different approaches and paths there are- Where they differ, where they converge.

In the end, you just have to put in the "BTU's" (Butt Time Units) and WRITE. As my screensaver reminds me when I stop writing for more than three minutes...

I always think that if you at least get something written, then you can always come back to it and change it. So the way I do things is write the whole thing, and then come back and change, and change until I get it to what I want. Is that one of the best ways?

Right, that's the philosophy behind "Don't get it right, get it written"

Most of writing, is re-writing. And my experience with optioning screenplays is that even when you feel it's "perfect" the people who put down the money will want changes. Your job is to make those changes work. And sometimes that can be almost impossible.

I took an intro screenwriting course recently and found its value half-educational, half-networking. The formatting part most of the class got through in the first two classes (though, amazingly enough, some people by the end still hadn't quite got the 'show me' don't 'tell me' part down).

What I found very useful was the three act structure, talking about dramatic points, techniques. It really made me watch films in a different way, structurally. Rhythm, pacing, payoffs. I think that part, the structural method of building a film story, was extremely useful. Some of it was what I had understood unconsciously when I analyzed films previously, but had trouble putting down in my own screenplay attempts.

Most of us have an unconcious grasp of Film syntax, that comes from the thousands of hours spent watching films. "FIlm is the literature of our generation" - Spielberg.

We instinctively know what works, and what doesn't... but often we don't know "why". This is the value of screenwriting courses and books. Understanding the inherent structure of film allows you to follow the standard, or re-arange it. It's the same old story in any art form, you have to "know the rules" to "break the rules".

The books and courses that stress a very specific structure "On page 3, the theme, on page 10 the exiting incident, on page 30 the threshold/act transistion..." Are useful in the way any template is usefull. (If you want to write a sonnet, you have to know it's form.) Personally, I write my scripts organically FIRST. After they are complete, I reread them for flow, and WHEN I sense a problem, I look at the templates and structure charts to see if I haven't missed something, or forced something. The templates are a handy reference, not HOLY WRIT - ETCHED IN STONE.

Sometimes I work from a treatment, sometimes I don't... A lot depends on how the idea comes to me, and how I feel about it.

Writing is such a subjective discipline... and quite lonely. That's another reason for classes and support groups as you pointed out... networking.

"There's nothing wrong with writing, as long as you do it in private and wash your hands afterwards." - Heinlein

"I always think that if you at least get something written, then you can always come back to it and change it. So the way I do things is write the whole thing, and then come back and change, and change until I get it to what I want. Is that one of the best ways?"

Yes and no, or more accurately, no and yes.

The real intent of writing a first draft is uncertainty. If you find yourself having a hard time critiquing your own ideas, then putting them on paper and showing them to someone else may provide fresh perspective.

If you know your idea isn't gold in the saucer, then why write it down?

The best films (books, songs, etc.) are built in layers--they're products of much retooling and are rarely the work of one person. I recently saw a talk by Pete Docter, the director of Monsters, Inc., and he shared the movie's concept as he pitched it (totally different from what showed up on screen) to the movie as it stood in a series of storyboards two years later (roughly half of all the elements of the final film were in place, and in the place of the half that had yet to be invented were completely different characters, subplots, and gags that were good but didn't entirely contribute to the unity and coherance of the piece).

Had those halfway-point storyboards never been drawn, and the animators went straight from script to screen--or heaven forbid, pitch to screen, as every Dreamworks movie to date seems to have been made--then the refinement that came from the input of so many people would have never taken place. Derogatively, call it "filmmaking by committee," which sounds contrary to the auteur spirit we independent filmmakers roar about. But, as Docter notes, movies don't happen when a writer jumps up in bed one night--"Dumbo!" Good movies result from the combined wealth of many talents.

But let's take the opposite case. You have some amount of uncertainty whether your collection of ideas qualify for a good film: novelty, audience appeal, and something I'll call "exceptionality" (for lack of a better antonym of "mediocrity"). You don't think your plot has drive and sure direction, your characters aren't crispy crystal clear in your mind, you're not absolutely positive how your movie ends. You haven't thought enough, outlined enough, researched enough or studied enough to source inspiration. You're not ready to write. Don't waste those butt units. You'll be putting down words that may constitute a screenplay, but not one worthy of transforming into a better screenplay through that committee process. (Now, whether that committee proccess is the purview of whole story and art departments or limited to just the voices in your head is up to your commitment to the concept of "singular vision." Just don't get tunnel vision.)

So if I were to some up my gist in a metaphor (as all good proselytizing idiot savants should)... The real idea here is the "kernel" or "seed" or "zygote." Every living being starts off as a tiny particle, yet has everything it needs to assimilate nutrition into an ever bigger, badder version of itself. Your movie will similarly find it easy to grow, once its seed is complete. It's up to you to evaluate whether your seed is ready for germination, but if you don't feel you could conduct both sides of an interview with each of your characters, or you couldn't tell them what their fates must be with the certitude of a palm reader, it's a good bet the kernel isn't there.

One other tip and a corollary to it. When you get stuck, throw in a new character with some new specialties. Endow the character with the magical ability to unstick everything. (This is OK because you're the one doing the writing!) Then work the character backwards and forwards in story-time to see if they're useful enough to be woven in at other points in the story. And remember, characters, like the gods and goddesses, heroes and monsters of classical myths, represent psychic functions, guiding values and perversions. Whether you choose to endow the character with a broad or narrow spectrum of values and perversions will determine the complexity of the character. Give the character just one symbolic value and you're left with stock; bequeath to him or her a variety of conflicting values and you give yourself something rich indeed to play with.

Going back to book suggestions, I highly recommend The Screenwriter's Bible. Not that I'm some expert, but as a long time writer in other forms, this book was a great introduction to established and not-so-established screenwriting methodologies.

The book is also of a very sensible viewpoint, and it's a viewpoint that I feel applies to learning any craft... First learn the rules, the rigidity, the formulas, etc., and then once you understand them, feel free to break them intelligently.

Just like in elementary school - sure a calculator can do math, but you still learn how to do it the old fashioned way first - on your fingers. It's all part of a well-rounded education because sometimes you have a calculator, sometimes you don't. You'd have an awful time trying to figure out if you qualify for the express lane at the market if you can't figure out without a calculator when you have 10 items or less.

Well I think the best way to go about screenwriting it just to start writing. Look at some sources some films you admirer and start there. But no matter what just write. Because you can always go back later and delete everything or just something.

This link below is to two great books by Ralph S Singleton.
I highly recommend these books. I have been using them and they will help prepare you in the next step after you write your script. Oh, If you don't follow the basic steps in his book you will most likely end up with difficulties in production.